Alkaline battery technology has been used since the 1970's to provide inexpensive, long-lasting portable power sources for a variety of electrical applications. Disposable alkaline batteries, or primary cells are the most common example. However, due to recent technical advancements, re-chargeable alkaline batteries, or secondary cells, have recently become available. These batteries are significantly less expensive for the end-user and are also more environmentally benign. In order to avoid gas production during recharging that could lead to dangerous internal overpressure and cell leakage, the chemistry of rechargeable alkaline batteries is significantly different from primary alkaline batteries. In order that rechargeable alkaline batteries are indistinguishable from primary alkaline batteries from a consumer's point of view, improvements need to be made to the cathode that result in improved electrical performance and that allow the batteries to be produced on conventional large-scale automated production equipment.
The cathode composition of primary alkaline batteries often comprises a binder that has the main task of increasing the flexural strength of the pressed cathode. Without the addition of sufficient binder (typically about 1.0-2.0% by weight) excessive pellet breakage occurs during automated production. The addition of a binder has an adverse impact on cell performance; since binders are electrochemically inactive, the presence of the binder reduces the quantity of active cathode components, such as manganese dioxide, that are available to participate in electrochemical reactions. Also, binders are typically non-conductive. The addition of a binder is therefore tolerated in primary alkaline batteries only because of its necessity for economic large-scale battery production.
There are many suitable binder materials. One such binder material is polyethylene powder, particularly the polyethylene powder manufactured under the trade name Coathylene® by the Swiss firm Herberts Polymer Powders SA (“Herberts”). In July, 1999, Herberts published a study entitled: “Coathylene® in Dry Cell Batteries”, and in November, 1998, another publication entitled: “Precipitated LDPE fine powders as binders in the manufacture of dry cell batteries”. These publications describe several advantages of Coathylene® powder when used in the cathode composition of a primary alkaline battery, such as: increased cathode strength, decreased mechanical friction, no significant decrease in conductivity, etc. These advantages are present when the binder material is added in a concentration of at least 1.0% and preferably 1.5 to 2.0% relative to the cathode mass. Lower concentration is not effective in achieving the aforementioned advantages.
Recent improvements in the chemistry of rechargeable alkaline battery cathode formulations have resulted in a desirable increase in cumulative discharge capacity, cycle life, and discharge current.
One example of an improved cathode formulation is disclosed in European patent EP0617845 B1, by Taucher, et al. Taucher discloses a rechargeable alkaline cell with barium compounds such as BaO, Ba(OH)2*8H2O and BaSO4 added to the cathode mix in the range of 3-25%. These barium compounds provide improved cumulative capacity, but also improve the flexural strength of the cathode, obviating the need for the addition of a binder. The addition of a binder to these cells is undesirable, since the presence of the binder reduces cell performance.
Another example of an improved cathode formulation is disclosed in U.S. Pat. No. 6,361,899, by Daniel-Ivad, et al., which is hereby incorporated by reference for jurisdictions that permit this method. Daniel-Ivad discloses a rechargeable alkaline cell in which the cathode includes hygroscopic additive compounds comprising oxides, hydroxides, or hydrates of barium or strontium. These hygroscopic additives desirably increase the performance of the cell, as indicated by increases in the cumulative discharge capacity and cycle life of the cell.
While both of the foregoing references disclose the use of certain barium compounds to improve cumulative cell capacity of rechargeable alkaline cells, neither reference contemplates continuous cell production nor addresses any of the issues that arise in a continuous production environment. In fact, when barium compounds such as BaO or Ba(OH)2*8H2O are used as additives, the formed pellets exhibit a “sticky” consistency that impedes continuous processing. As a result, the improved cathode formulations cannot be used in a continuous production environment, making batteries with these formulations too expensive for the end-user. Although binders are used to increase flexural strength of the cathode as an aid in continuous pellet processing, binders are not normally selected to modify the consistency of the cathode pellet. In fact, “stickiness” is not a problem for primary alkaline batteries, as the cathode pellets in these cells do not contain hygroscopic additives.
Accordingly, there is still a need for an improved rechargeable alkaline battery cathode composition that results in increased battery performance while permitting manufacturing in a continuous production environment.
Rechargeable alkaline cells are prone to cell failures when overcharging takes place. Overcharge results in damage to cell components and will cause increased internal gassing, which in turn may eventually cause the cell to fail from electrolyte leakage due to overpressure. The critical voltage limit above which damage can occur is reported at 1.68V in the literature (D. Linden, Handbook of Batteries, 2nd Edition, Mc-Graw Hill, New York, 1995). Typically, commercial chargers are voltage limited at about 1.65V, which is the generally accepted safe value for rechargeable alkaline cells. However, a portion of the cell's capacity is not re-charged, which results in a loss of available performance. The loss of available performance is especially significant when inert materials such as additives or binders are added to the cathode, as the presence of these materials reduces the amount of active cathode component available for electrochemical reaction.
The need therefore exists for an improved charging method for rechargeable alkaline cells, particularly cells containing additives and binders.